The present invention relates to a copying machine which can vary a copying density.
In copying machines, a copying density is automatically adjusted so as to obtain an optimum density regardless of a density of a document. In other words, an exposure amount of the document and/or a bias voltage of a developer is changed in accordance with the density of the document, thereby controlling the copying density so as not to be affected by the density of the document. However, when an optimum copy cannot be obtained by such an automatic adjustment, or when a copy having a desired density different from the optimum one is needed, copying machines which can manually vary the copying density are used.
One conventional density setting means consists of a slide lever arranged on an operation panel and a variable resistor which is operated in synchronism with the lever. According to this means, the copying density can be set in a non-step manner and fine adjustment can be performed. However, this means cannot be electrically set and therefore has the following disadvantage. Since the previous setting value remains unchanged, a setting error easily occurs when the operation mode is changed from the interrupt copying mode to the normal copying mode. Further since the non-step adjustment can be performed, a user is confused as to where the density should be set. Another setting means consisting of three keys of "dark", "normal" and "light" is also known. According to this means, although this means can be electrically set, this means has only three setting values and fine adjustment cannot be performed.